Optical transmission systems, such as long-haul undersea optical transmission systems, may be used to transmit optical signals over long distances. These long-haul optical transmission systems, however, suffer from signal degradation caused by many factors, for example, losses due to thermal noise and scattering caused by optical fiber imperfections as well as losses resulting from splicing during assembly. These and other factors combine to attenuate the optical signal propagating through the transmission system.
To address this problem, optical signals are optically amplified at predetermined locations along the transmission system. Optical transmission systems may include repeaters connected to lengths of fiber optic cable. The repeaters may include optical amplifiers for amplifying optical signals transmitted in each direction in the transmission system. A repeater together with a length of fiber optic cable generally forms a transmission span, and multiple transmission spans form an optical transmission segment. A system may be designed such that the amplification provided by each repeater (i.e., the repeater gain) compensates for the signal loss in the preceding transmission span (i.e., span loss).
In existing optimal transmission segment designs, all of the spans are generally designed to have the same nominal values with respect to gain, gain shape and noise contribution. For example, all amplifiers may be identical and all cable lengths have the same nominal loss. The repeaters may be designed to yield a flat gain across a given wavelength range for each transmission span. In existing optimized transmission segments, therefore, the nominal gain of each repeater is ideally equal to the nominal loss of each cable length to provide a net gain of about zero.
To reduce cost in traditional undersea optical transmission systems, repeaters were custom designed to support the longest possible repeater spacing consistent with performance and capacity requirements for the proposed transmission segment. One result of this approach has been the proliferation of repeater gain codes, with each new transmission segment design resulting in a new gain code that is optimized for that particular segment design. More recently, transmission systems have been constructed to make efficient use of existing inventory to meet customer capacity, schedule and performance requirements. When using inventory repeaters, however, the resulting segment designs may be suboptimal, either in repeater count or in segment gain shape.
Imbalances between repeater gain and span loss have been a problem in systems built from repeaters in inventory with stretched repeater spacing as well as in new systems despite best efforts to match repeater gain and span loss. During system assembly, for example, uncertainty in splicing losses and the need to accommodate cables with losses different than the nominal design loss can result in net gains significantly offset from the ideal zero net gain for a transmission system. Imbalances may also be caused by losses added during system repairs, for example, by adding extra cable and splices.
The imbalance between repeater gain and span loss may detrimentally affect the optical signal quality. In particular, when the span losses in the assembled transmission spans exceed the repeater gain, negative gain tilt may occur. As used herein, gain tilt is the difference (e.g., in dB) between the highest channel power and the lowest channel power for a given wavelength range. Negative gain tilt may adversely affect the optical signal to noise ratio (OSNR) of the communication system, may consume dynamic range in pre-emphasis, and may compromise the operation of a line monitoring system (LMS).
One solution to this problem includes monitoring gain tilt and managing gain tilt by adding line build-out attenuators (LBOs) to the optical path in couplings or joints or by adding tilt filters in a gain equalization joint (GEJ) as needed to maintain system gain tilt within acceptable limits. A LBO may be added when the measured gain tilt is positive and a GEJ may be added when the measured gain tilt is negative. When a GEJ is added, the cable span between repeaters may need to be shortened to compensate for the span loss resulting from the insertion of the GEJ itself. Alternatively, when a GEJ is inserted into a nominal loss span, loss is added to the span loss, which introduces negative gain tilt. Thus, adding a GEJ to a transmission system adds loss, which degrades OSNR, and increases cost.
Accordingly, there is a need for a method of managing gain tilt in an optically amplified transmission system using a limited number of non-optimum repeaters. There is also a need for simplified design and manufacturing of an optical communication system using a limited number of repeater codes.